1. Field of the Invention
The present invention relates to an air conditioner in which two compressors are connected in parallel with a refrigerant circuit of one system.
2. Description of Relevant Art
As a conventional apparatus of this type, one shown in FIG. 53A (or 53B) is known. In the drawing, reference character A denotes a heat source unit, and B denotes an indoor unit. Reference numeral 1 (or 201) denotes a first compressor of a low-pressure shell type; 2 (or 202), a second compressor of the low-pressure shell type; and 3 (or 203), an equalizing pipe for connecting together the shell of the first compressor 1 and the shell of the second compressor 2, the equalizing pipe 3 being disposed at a position sufficiently higher than a minimum oil level for properly effecting the lubrication of the compressors. Numeral 4 denotes a discharge pipe of the first compressor 1; 5, a discharge pipe of the second compressor 2; 6, a common discharge pipe provided after the discharge pipes 4, 5 converge; 7 (or 212), a suction pipe of the first compressor 1; 8 (or 213), a Auction pipe of the second compressor 2; 9, a common suction pipe before branching into the suction pipes 7, 8; 10 (or 204), an oil separator provided in the common discharge pipe 6 and having a shell 10a, an inlet pipe 10b, an outlet pipe 10c, and an oil return pipe 10d; 11 (or 205), a four-way changeover valve; 12 (or 206), a heat source unit-side heat exchanger; 15 (or 209), an accumulator provided in a branching portion in which the common suction pipe 9 branches into the suction pipes 7, 8; 22, an oil-returning bypass passage for connecting the oil return pipe 10d of the oil separator 10 and the common suction pipe 9; 23 (or 210), a solenoid on-off valve provided midway in the oil-returning bypass passage 22; and 24 (or 211), a capillary tube provided in parallel with the solenoid on-off valve 23. Numeral 16 denotes a U-pipe provided in the accumulator 15 and corresponding to the suction pipe 8, and numeral 17 denotes a U-pipe provided in the accumulator 15 and corresponding to the suction pipe 8. Numeral 18 denotes a bypass hole provided in the U-pipe 16 and designed to prevent the first compressor 1 from becoming damaged by temporarily sucking lubricating oil and a liquid refrigerant 25 accumulated in the U-pipe 16 at the time of the starting of the first compressor 1. Numeral 19 denotes a bypass hole provided in the U-pipe 17 and designed to prevent the second compressor 2 from becoming damaged as the second compressor 2 temporarily sucks lubricating oil and the liquid refrigerant 25 accumulated in the U-pipe 17 at the time of the starting of the second compressor 2. Numeral 20 denotes an oil return hole provided in the U-pipe 16 for gradually sucking the lubricating oil and the liquid refrigerant 25 accumulated in the bottom of the accumulator 15 and returning the same to the first compressor 1. Numeral 21 denotes an oil return hole provided in the U-pipe 17 for gradually sucking the lubricating oil and the liquid refrigerant 25 accumulated in the bottom of the accumulator 15 and returning the same to the second compressor 2. The heat source unit A is arranged as described above. Numeral 13 (or 207) denotes a throttling device; 14, an indoor-side heat exchanger; and B, an indoor unit comprised of the aforementioned throttling device 13 and the indoor-side heat exchanger 14. Numeral 26 denotes a first connecting pipe having one end connected to the heat source unit A by the heat source unit-side heat exchanger 12 and the other end connected to the indoor unit B by the throttling device 13, while numeral 27 denotes a second connecting pipe having one end connected to the heat source unit A by the four-way changeover valve 11 and the other end connected to the indoor unit B by the indoor-side heat exchanger 14. In the drawing, the solid-line arrows indicate the direction of flow of the refrigerant during cooling operation, while the broken-line arrows indicate the direction of flow of the refrigerant during heating operation.
Next, a description will be given of the operation during cooling operation. The high-temperature, high-pressure gas refrigerant discharged from the first compressor 1 or the second compressor 2 passes through the oil separator 10 and the four-way changeover valve 11, and flows into the heat source unit-side heat exchanger 12 where the gas refrigerant radiates heat and condenses into a high-pressure liquid refrigerant. The pressure of this liquid refrigerant is reduced by the throttling device 13, and flows into the indoor-side heat exchanger 14 as a low-pressure gas-liquid two-phase refrigerant. By absorbing heat here, the refrigerant evaporates, flows into the accumulator 15 via the four-way changeover valve 11, passes thorough the U-pipes 16, 17 and the suction pipes 7, 8, and returns to the first compressor 1 or the second compressor 2.
At this time, as for the lubricating oil which has flowed out together with the refrigerant from the first compressor 1 or the second compressor 2, a major portion of it is separated by the oil separator 10, and is accumulated in the shell 10a of the oil separator 10. A portion of the accumulated lubricating oil, together with the gas refrigerant in the oil separator 10, is constantly sent to the accumulator 15 via the common suction pipe 9 by the capillary tube 24. The remaining lubricating oil in the shell 10a of the oil separator 10 is sent to the accumulator 15 via the common suction pipe 9 as the solenoid on-off valve 23 is opened. The lubricating oil which was not separated by the oil separator 10 is sent together with the refrigerant to the accumulator 15 via the four-way changeover valve 11, the heat source unit-side heat exchanger 12, the throttling device 13, the indoor-side heat exchanger 14, and the four-way changeover valve 11. The lubricating oil which has entered the accumulator 15 is accumulated in the bottom of the accumulator 15, and a portion of it flows into the U-pipes 16, 17 through the oil return holes 20, 21, passes through the suction pipes 7, 8, and returns to the first compressor 1 or the second compressor 2.
Since the first and second compressors 1, 2 are of the low-pressure shell type, the following relationships hold among the pressure P.sub.S0 at a branching portion where the common suction pipe 9 branches into the suction pipes 7, 8, the pressure P.sub.S1 within the shell of the first compressor 1, and the pressure P.sub.S2 within the shell of the second compressor 2: EQU P.sub.S1 =P.sub.S0 -.DELTA.P.sub.S1 EQU P.sub.S2 =P.sub.S0 -.DELTA.P.sub.S2
where .DELTA.P.sub.S1 is a pressure loss from the branching portion where the common suction pipe 9 branches into the suction pipes 7, 8 to the first compressor 1, while .DELTA.P.sub.S2 is a pressure loss from the branching portion where the common suction pipe 9 branches into the suction pipes 7, 8 to the second compressor 2, and the following relationships hold: EQU .DELTA.P.sub.S1 =z.sub.1 r.sub.g V.sub.12 /2 EQU .DELTA.P.sub.S2 =z.sub.2 r.sub.g V.sub.22 /2
r.sub.g : concentration of the gas refrigerant PA1 V.sub.1 : flow rate of the gas refrigerant flowing through the suction pipe 7 PA1 V.sub.2 : flow rate of the gas refrigerant flowing through the suction pipe 8 PA1 z.sub.1 : constant representing channel resistance from the branching portion where the common suction pipe 9 branches into the suction pipes 7, 8 up to the first compressor 1 PA1 z.sub.2 : constant representing channel resistance from the branching portion where the common suction pipe 9 branches into the suction pipes 7, 8 up to the second compressor 2 PA1 r.sub.1 : concentration of a mixture of the lubricating oil and the liquid refrigerant PA1 g: gravitational acceleration PA1 h.sub.1 : liquid level of the first compressor 1 with respect to the connecting portion between the shell of the first compressor 1 and the equalizing pipe 3 (when the liquid level is lower than the connecting portion, a setting is provided such that h.sub.1 =0) PA1 h.sub.2 : liquid level of the second compressor 2 with respect to the connecting portion between the shell of the second compressor 2 and the equalizing pipe 3 (when the liquid level is lower than the connecting portion, a setting is provided such that h.sub.2 =0) PA1 G.sub.1 =a(.DELTA.p/r).sup.1/2 PA1 G.sub.1 : flow rate of a mixed liquid of refrigerant and lubricating oil flowing through the equalizing pipe 3 PA1 .alpha.: flow coefficient PA1 .rho.: concentration of the mixed liquid PA1 .DELTA.p: differential pressure across the equalizing pipe (=difference between internal pressures of compressor shells)
Accordingly, a pressure difference .DELTA.P.sub.S12 (P.sub.S1 -P.sub.S2), which is shown below, takes place in the shells of the first compressor 1 and the second compressor 2. EQU .DELTA.P.sub.S12 =(--z.sub.1 V.sub.12 +z.sub.2 V.sub.22) r.sub.g /2
Therefore, the pressure difference .DELTA.P which occurs at both ends of the equalizing pipe 3 becomes as shown below. It should be noted, however, that it is assumed that both ends of the equalizing pipe are at substantially the same height. EQU .DELTA.P=.DELTA.P.sub.S12 +r.sub.1 g (h.sub.1 -h.sub.2)
That is, when .DELTA.P&gt;0, the gas refrigerant and a mixed liquid (a mixed liquid of the lubricating oil and the liquid refrigerant) flow from the first compressor 1 to the second compressor 2 through the equalizing pipe 3. Meanwhile, when .DELTA.P&lt;0, the gas refrigerant and the mixed liquid (the mixed liquid of the lubricating oil and the liquid refrigerant) flow from the second compressor 2 to the first compressor 1 through the equalizing pipe 3.
In addition, when .DELTA.P&gt;0, the liquid level of the mixed liquid (the mixed liquid of the lubricating oil and the liquid refrigerant) in the first compressor 1 drops until it reaches the position of the equalizing pipe 3, but it does not drop further below that position. Hence, if the concentration of the lubricating oil is high, the lubrication of the second compressor 2 is effected properly. Thus, a refrigeration cycle during cooling is formed.
Next, a description will be given of the operation during heating operation. The high-temperature, high-pressure gas refrigerant discharged from the first compressor 1 or the second compressor 2 passes through the oil separator 10 and the four-way changeover valve 11, and flows into the indoor-side heat exchanger 14 where the gas refrigerant radiates heat and condenses into a high-pressure liquid refrigerant. The pressure of this liquid refrigerant is reduced by the throttling device 13, and flows into the heat source unit-side heat exchanger 12 as a low-pressure gas-liquid two-phase refrigerant. By absorbing heat here, the refrigerant evaporates, flows into the accumulator 15 via the four-way changeover valve 11, passes thorough the U-pipes 16, 17 and the suction pipes 7, 8, and returns to the first compressor 1 or the second compressor 2.
At this time, as for the lubricating oil which has flowed out together with the refrigerant from the first compressor 1 or the second compressor 2, a major portion of it is separated by the oil separator 10, and is accumulated in the shell 10a of the oil separator. A portion of the accumulated lubricating oil, together with the gas refrigerant in the shell 10a of the oil separator, is constantly sent to the accumulator 15 via the common suction pipe 9 by the capillary tube 24. The remaining lubricating oil in the shell 10a of the oil separator 10 is sent to the accumulator 15 via the common suction pipe 9 as the solenoid on-off valve 23 is opened. The lubricating oil which was not separated by the oil separator 10 is sent together with the refrigerant to the accumulator 15 via the four-way changeover valve 11, the indoor-side heat exchanger 14, the throttling device 13, the heat source unit-side heat exchanger 12, and the four-way changeover valve 11. The lubricating oil which has entered the accumulator 15 is accumulated in the bottom of the accumulator 15, and a portion of it flows into the U-pipes 16, 17 through the oil return holes 20, 21, passes through the suction pipes 7, 8, and returns to the first compressor 1 or the second compressor 2.
Since the flow through the equalizing pipe 3 is utterly the same as during cooling, description thereof will be omitted here. Thus, a refrigeration cycle during heating is formed.
The flow-rate of a mixed fluid flowing through the equalizing pipe 3 is calculated in simple form by the following formula:
With such a conventional air conditioner, after the power supply of the air conditioner is turned on, in a state in which, of the two compressors 1 and 2, the compressor 1 is being operated and the compressor 2 is being stopped, when the compressor 2 is started after the running capacity of the compressor 1 has reached a certain level, the compressor 2 is started as it is.
Then, a description will be given of the case where the liquid refrigerant is accumulated in the accumulator 15 and of the operation of the refrigerant system at that time. When the first compressor 1 is started in a state in which both the first compressor 1 and the second compressor 2 are being stopped, since the evaporation temperature of the refrigerant in the evaporator (the indoor-side heat exchanger 14 during cooling operation and the heat source unit-side heat exchanger 12 during heating operation) has not been sufficiently lowered, the unevaporated liquid refrigerant temporarily flows into the common suction pipe 9. However, since the accumulator 15 is provided, the liquid refrigerant which has been sucked in the state of wet vapor does not reach the first compressor 1 or the second compressor 2, is temporarily stored in the accumulator 15, flows into the U-pipe 16 through the oil return hole 20 together with the lubricating oil accumulated here, and returns gradually to the first compressor 1 via the suction pipe 7. For this reason, the first compressor 1 is prevented from being damaged by the temporary wet vapor suction at the time of starting. In addition, since the quantity of wet vapor sucked at that time is not very large, the liquid refrigerant in the accumulator 15 is removed in a relatively short time.
In addition, when the first compressor 1 is started in a state in which the both the first and second compressors 1 and 2 have been stopped for a long time, the first compressor 1 is started in the state in which a large quantity of refrigerant lies inside the shells of the first and second compressors 1 and 2 as a liquid refrigerant. In this case, the liquid refrigerant held up inside the shell of the first compressor 1 is discharged in the form of a saturated gas or partially in the liquid state as it is, and the liquid refrigerant flows into the oil separator 10 via the discharge pipe 4 and the common discharge pipe 6. Since the discharge pipe 4, the common discharge pipe 6, and the oil separator 10 have become cool by being cooled by the outside air during stopping for a long time, the saturated gas refrigerant discharged from the first compressor 1 is cooled, condensed and liquefied. In addition, since the first compressor 1 is operated and the second compressor 2 remains stopped, the pressure within the shell of the first compressor 1 is lower than the pressure within the shell of the second compressor 2, and the liquid refrigerant held up inside the shell of the second compressor 2 is supplied to the first compressor 1 via the equalizing pipe 3. In the same way as the liquid refrigerant held up inside the shell of the first compressor 1, this liquid refrigerant is discharged in the form of a saturated gas or partially in the liquid state as it is, and this liquid refrigerant flows into the oil separator 10 via the discharge pipe 4 and the common discharge pipe 6, while the saturated gas refrigerant is cooled, condensed and liquefied. In the oil separator 10, a major portion of the liquid refrigerant is separated, and flows into the common suction pipe 9 via the solenoid on-off valve 23 since the solenoid on-off valve 23 is open for a fixed period of time during starting. However, since the accumulator 15 is provided, the liquid refrigerant which has been sucked in the state of wet vapor does not reach the first compressor 1 or the second compressor 2, is temporarily stored in the accumulator 15, flows into the U-pipe 16 through the oil return hole 20 together with the lubricating oil accumulated here, and returns gradually to the first compressor 1 via the suction pipe 7. For this reason, the first compressor 1 is prevented from being damaged by the temporary, but a large quantity of, wet vapor suction at the time of starting after stopping for a long time. In addition, since the quantity of wet vapor sucked at that time is very large, the liquid refrigerant in the accumulator 15 is removed after the lapse of a relatively long time.
During the cooling operation the first connecting pipe 26 is in the high-pressure liquid single phase or in the gas-liquid two-phase state in which the dryness is very small, but during the heating operation the first connecting pipe 26 is in the gas-liquid two-phase state in which the dryness is 0.1 to 0.2. Hence, the average concentration of the refrigerant in the first connecting pipe 26 is much greater during the cooling operation than during the heating operation, so that the quantity of refrigerant distributed in the first connecting pipe 26 is larger during the cooling operation. Accordingly, in a case where the locations of installation of the heat source unit A and the indoor unit B are distant from each other and the first connecting pipe 26 is long, the total quantity of refrigerant required during the cooling operation becomes greater than the total quantity of refrigerant required during the heating operation. Since the quantity of refrigerant charged in the system is normally determined by the operation in which the total quantity of refrigerant required becomes maximum, excess refrigerant is produced during the heating operation by a portion in which the total quantity of refrigerant required is smaller than during the cooling operation. This excess refrigerant is distributed in the accumulator 15.
In addition, if the first connecting pipe 26 is short, excess refrigerant is produced in the case of a chargeless system in which the refrigerant is not added when the setup work of the system is carried out by fixing the quantity of refrigerant charged in the system irrespective of the length of the first connecting pipe 26, i.e., by setting the quantity of refrigerant charged in the system to be the total quantity of refrigerant required when the length of the first connecting pipe 26 is the largest irrespective of the length of the first connecting pipe 26. This excess refrigerant is distributed in the accumulator 15.
As described above, at the time when the power is turned on, there is a high possibility that a large quantity of refrigerant was held up in the compressors before then. When the compressor 1 is started, the lubricating oil is also liable to flow out together with the liquid refrigerant owing to foaming at the time of starting, so that the quantity of lubricating oil in the compressor 1 becomes small. When only the compressor 1 is being operated, since the internal pressure of the shell of the compressor 2 being stopped is higher than the internal pressure of the shell of the compressor 1 being operated, the lubricating oil in the compressor 2 is supplied together with the refrigerant to the shell in the compressor 1. In addition, since oil is not returned from the accumulator 15 to the compressor 2 being stopped, the level of the mixed liquid of the lubricating oil and the refrigerant in the compressor 2 drops to the vicinity of the height of the equalizing pipe 3.
When the compressor 2 is started in such a stopped state, the refrigerant undergoes foaming in the compressor 2 due to a sudden drop in the internal pressure of the shell during starting, the lubricating oil in the compressor 2 is liable to be discharged together with the refrigerant, and the pressure difference across the equalizing pipe 3 becomes small or is reversed. Hence, the quantity of lubricating oil supplied to the compressor 1 through the equalizing pipe 3 decreases.
In addition, an observation is made of the compressor 2 which is started, in a case where its running capacity is smaller than that of the compressor 1 already in operation, the relative magnitude of the pressure within the shell is higher in the case of the compressor 2. As a result, when the compressor 2 is started, the liquid level in the shell of the compressor 2 rises with foaming, and even though the liquid level may be lower than the equalizing pipe 3 during stopping, the liquid level rises above the height of the equalizing pipe after starting, so that the mixed liquid of the refrigerant and the lubricating oil is liable to flow out from the equalizing pipe 3 toward the compressor 1.
In a case where the running capacity of the compressor 2 which is started is higher than that of the compressor 1 being operated, the internal pressure of the shell becomes higher in the case of the compressor 1. Hence, oil is returned to the compressor 2 from the compressor 1 through the equalizing pipe 3, but when the concentration of the lubricating oil in the compressor 1 which is already in operation is low, such an oil-returning effect is small.
In a state in which the liquid refrigerant is being accumulated in the accumulator 15, even when the compressor 1 is being operated and the compressor 2 is being stopped, since the interior of the shell of the compressor 1 and the interior of the shell of the compressor 2 communicates with each other via the equalizing pipe 3, the internal pressure of the shell of the compressor 2 being stopped also drops. Consequently, wet gas refrigerant moves from the accumulator 15 to the compressor 2, and condenses in the compressor 2, so that the concentration of the lubricating oil in the compressor 2 decreases gradually. For this reason, in a case where the compressor 2 is started for the first time after the power is turned on, there has been a risk that a bearing is damaged due to a shortage of the lubricating oil. In addition, in a case where excess liquid refrigerant is accumulated in the accumulator 15 when the compressor 2 being stopped is started, wet vapor suction has been liable to occur.
In addition, in a case where the compressor 1 is started with the refrigerant being held up in both compressors, the concentration of the lubricating oil in the compressor 1 is low, and the lubricating oil is liable to flow out due to foaming during starting. Yet, when the compressor 2 is being stopped, the mixed liquid of the refrigerant and the lubricating oil in the compressor 2 is also supplied to the compressor 1 through the equalizing pipe 3. However, if the compressor 2 is started in a short time after the starting of the compressor 1, the lubricating oil in the compressor 2 is discharged together with the refrigerant due to foaming, and it becomes difficult for the oil to be returned to the compressor 1 from the equalizing pipe 3. Furthermore, since the quantity of lubricating oil in the compressor 1 is not sufficient, it is difficult to expect the return of oil from the compressor 1 through the equalizing pipe 3 to the compressor 2 which was started. Hence, there has occurred a problem of seizure of the bearings of the compressors due to the shortage of the lubricating oil in the compressor 1 and the compressor 2.
Furthermore, since the conventional air conditioner is arranged as described above, in a case where the first compressor 1 is being operated and the second compressor 2 is being stopped, the refrigerant flows into the first compressor 1 not only via the U-pipe 16 and the suction pipe 7 but also via the U-pipe 17, the suction pipe 8, the shell of the second compressor 2, and the equalizing pipe 3. At this time, if the liquid refrigerant is accumulated in the accumulator 15, the liquid refrigerant is also accumulated inside the U-pipe 17 due to the presence of the oil return hole 21. Hence, since the flow rate of the refrigerant supplied from the bypass hole 19 is insufficient as the flow rate of the refrigerant supplied to the first compressor via the shell of the second compressor 2, and the liquid refrigerant accumulated inside the U-pipe 17 flows into the shell of the second compressor 2. As a result, the lubricating oil in the shell of the second compressor 2 is mixed with the liquid refrigerant, which has flowed in, and flows out to the shell of the first compressor 1 via the equalizing pipe 3 since the pressure within the shell of the first compressor 1 is lower than the pressure within the shell of the second compressor 2. Thus, the lubricating oil in the second compressor 2 declines in terms of its absolute quantity while the first compressor 1 is being operated and the second compressor 2 is being stopped, and the concentration of the lubricating oil also declines. Consequently, there has been a problem in that a shortage of lubricating oil or faulty lubrication due to such as the lack of viscosity of the lubricating oil occur when the second compressor is started, possibly resulting in the breakage of the second compressor.